Soil Horizons

video notes

Since humus forms are comprised of a group of soil horizons located at or near the surface, which have formed from organic residues (separate from or mixed with mineral particles), here we provide more details about those horizons that are included in a humus form, namely organic horizons L, F, and H, and a mineral Ah horizon.

According to the Soil Classification Working Group (1998), organic horizons contain more than 17% organic C (or more than 30% of organic matter) by weight. Organic horizons occur in Organic soils or they may be present at the surface of mineral soils. There are two groups of organic horizons – those that are formed in relatively well-drained conditions (LFH) and those that are formed in poorly drained conditions (O).

Well-Drained Conditions

Under well-drained (upland) conditions, organic horizons are composed of mostly leaves or needles, twigs and woody material and may include: L, F and H horizons. Tree litter falls to the forest floor and acts as a source of food for soil organisms. As organisms decompose litter, long-lasting humic compounds are formed and the litter becomes more discoloured and fragmented, over time shifting from litter (L) to humic (H) material.

Organic horizon descriptions and designations shown on this page follow taxonomic classification derived by Green et al. (1993) and mineral horizon descriptions follow those presented by the Soil Classification Working Group (1998).

L ( Litter )

litter

Litter – Relatively fresh organic residues, identifiable plant material, such as leaves, wood or twigs resting on the surface of the forest floor. Some discolouration or other signs of early decomposition may be visible, but the origins of plant residues are still easy to discern.

Alder Litter Maple Litter Needles Close

From left to right,litter originating from red alder, bigleaf maple, western hemlock and Douglas-fir

 

ln (new) – Recently fallen material, mostly unfragmented and loose plant residues showing no structural change, may be somewhat discoloured.
lv (variative) – Less recently fallen material, disintegration and discolouration have occurred, but fragmentation and fine substances are lacking.

F ( Fermented )

fermented

F (Fermented, fibric, fragmented) – Decomposition of plant material is apparent, but the origins of plant residues are still distinguishable. Often, roots are present.

fm (mycogenous) – Plant residues are aggregated in a matted structure with a tenacious consistence (difficult to pull apart). Abundant fungal mycelia and roots, but few or no faunal droppings.

Fm
(Fm): After peeling the forest floor from the mineral soil, yellow and white fungal mycelia were clearly visible.
fz (zoogenous) – Plant residues are weakly aggregated with a loose friable consistency due to faunal droppings and active mixing of organic and mineral material by fauna. Few fungal mycelia if present. Faunal droppings are typically numerous and easily observed under magnification with a hand lens. Root residues are present, but are less abundant than in Fm horizons.

Fz Castes
(Fz): Caterpillar casts in the Fz
fa (amphi) – Plant residues are aggregated into a weak to moderate, noncompact matted structure. Fabric is variable, featuring clumps of aggregated material with pockets of loose material. This is an intergrade between Fm and Fz horizons. There is presence of fungal mycelia and/or faunal droppings, but neither in dominance over the other.

H ( Humic )

humic
H (Humic) – Well-humified plant material so that plant residues are not recognizable, with the exception of some roots or wood. This material is in advanced stage of humification in which fine substances predominate over plant residues.

H
Organic material in an H horizon has been humified to the point that the origins of the material are no longer recognizable.
Hh (humic) – Fine substances with few or no recognizable plant residues, dark colour, greasy (stains fingers) when moist; massive or blocky structure.
Hz (zoogenous) – Fine substances with few or no recognizable plant residues, dominated by faunal droppings, dark colour and granular structure. Abundant very small cylindrical or spherical faunal droppings gives appearance of fine black ‘sawdust’.
Hr (residues) – Fine substances with some recognizable plant residues (wood, bark or roots), slightly greasy when moist and it does not stain fingers with dark colors when rubbed. Dark reddish-brown with hues around 2.5YR, and are at least one or two hues redder than underlying Hh horizons, if present.

Poorly Drained Conditions

Where soils are saturated for prolonged periods decomposition is limited by low oxygen availability. Thick organic horizons develop primarily from mosses and woody materials and are classified as Organic soils in the Canadian System of Soil Classification. The organic horizons in poorly drained Organic soils are classified as O horizons.

O ( Organic )

organic

O (Organic) – Organic material, variably decomposed and influenced by a water table near the soil surface for extended time periods. Associated with wetlands where water table is at or near soil surface for a significant amount of time during frost-free periods.

of Identifiable plant residues (poorly decomposed).
om Plant residues are partly decomposed. This is an intermediate extent of decomposition between Of and Oh horizons.
oh Well-decomposed plant residues, largely transformed into humic materials.

Mineral Horizons

Below the forest floor there may be a mineral soil horizon that is rich in organic material, but with <30% organic matter (or <17% of organic C) by weight, and dark in colour.

A ( Mineral )

mineral

A (Mineral) – Mineral horizon formed at or near the soil surface, containing less than 17% organic C by weight.

ah the only mineral horizon included in humus form classification. The colour value is at least 1 unit lower than the horizon below, or at least 0.5% more carbon than the IC horizon or both.

Ah
(Ah): Soil fauna have mixed organic material with mineral soil to form an Ah with a granular structure

 


Lowercase suffixes – can be applied to any organic horizon (p and u also applied to Ah)

i – (intermixed): contains intermixed mineral particles finer than 2 mm, with 17-35% organic C by weight. Mixing may result from different processes (colluvial, eolian, alluvial, cryoturbation, silvoturbation, zooturbation)

p – (plowed): substantially altered by anthropogenic activities (e.g., logging, site preparation). Type of activity should be noted.

u – substantially altered by natural processes (e.g., blowdown of trees, animal burrowing, erosional events). Type of process should be noted.

w – (wood): contains significant amounts (>35% of volume of solids) of coarse woody debris in various stages of decomposition. Used with subordinate horizons of F and H master horizons (e.g., Fmw) to denote a significant component of decaying wood. Fw and Hw are comprised entirely of partially and well-humified decaying wood. They typically are red colored with high chromas and lack dark-colored humic substances which originate from nonligneous plant residues.

y – a horizon affected by cryoturbation (associated with permafrost)